It is common to employ actuator-controlled valves in safety-critical industrial systems, such as the oil & gas, power generation and marine industries. Automated safety critical valves can be designed to operate as part of a closed loop system to prevent dangerous plant conditions from developing, for example opening to release a built-up overpressure, closing to shut down a specific line, or divert a hazardous process stream into a holding tank, or the like.
Such valves need to react immediately when called upon, but may be left uncalled-for and therefore in a fixed position for an extended period of time. Accordingly, there is a need to verify from time to time that the valve will function correctly during a safety demand. However, a full test of the valve would result in the plant in which it is fitted being shut down whilst the test is performed. This is intrinsically undesirable as it will disrupt the operation of the plant and reduce its productivity.
For this reason, so-called “Partial Stroke Testing” (PST) is employed in order to enable testing of the valve to occur without necessitating a plant shut down. PST involves moving a valve in the direction that it is required to move during a safety demand, but without fully operating it. This allows the valve to be tested without necessitating a plant shut down. WO99/21066 discloses a system of this type, useful for pneumatically-controlled valves.